(1) Field of the Technology
The present technology relates to an image forming apparatus based on electrophotography using a dual-component developing system.
(2) Description of the Prior Art
In image forming apparatus based on electrophotography, which are often applied to copiers, printers, facsimile machines, etc., a photoreceptor drum having a photosensitive layer containing photoconductive substances formed on the surface thereof is used as an image bearer, the surface of the photoreceptor drum is uniformly electrified by imparting electric charge, then an electrostatic latent image corresponding to image information is formed using various image forming processes. This electrostatic latent image is developed by supplying a developer containing a toner from a developing roller or the like to form a toner image, which in turn is directly transferred to a recording medium such as paper etc. Alternatively, the toner image may be once transferred to an intermediate transfer element (which will be referred to hereinbelow as “primary transfer”), then the image is transferred from intermediate transfer element to a recording medium (which will be referred to hereinbelow as “secondary transfer”). The toner image thus secondarily transferred is usually fixed to the recording medium by a fusing means.
FIG. 1 is a schematic view showing a configuration of a photoreceptor drum 51, a developing unit 54 and therearound. A developing roller 57 supported by a developing hopper 56 of developing unit 54 is arranged a clearance away from photoreceptor drum 51. Developing unit 57 includes a multi-pole magnetic member 65 having multiple magnetic poles and a non-magnetic sleeve 66 which is rotatably fitted on multi-pole magnetic member 65. Multi-pole magnetic member 65 is a so-called magnet roller having a plurality of magnetic poles 71 to 75 radially arranged apart from each other. A dual-component developer 64 containing toner and carrier is supported on the developing roller 57 surface and conveyed by the magnetic force generated by these magnetic poles 71 to 75. Main magnetic pole 71 arranged at the proximal position between developing roller 57 and photoreceptor drum 51 form a magnetic brush of dual-component developer 64 by its magnetic force so as to form a developing nip area in which dual-component developer 64 is in contact with photoreceptor drum 51.
In the nip area, the toner is attracted to the electrostatic latent image on the photoreceptor drum 51 surface and transfers to the photoreceptor drum 51 surface to develop the electrostatic latent image. The carrier passing by the developing nip area will not adhere to photoreceptor drum 51 surface but returns into developing hopper 56.
A regulating member 58 is disposed at a position near the opening mouth of developing hopper 56 and on the upstream side with respect to the conveyed direction of the developer by developing roller 57, and an agitating member 59 is provided inside developing hopper 56.
FIG. 2 is a schematic diagram showing lines of magnetic force around conventional photoreceptor drum 51, developing roller 57 and therearound. The lines of magnetic force are formed from main magnetic pole 71 toward adjacent magnetic poles 72 and 73 of the opposite polarity. The magnetic flux density becomes maximum at the center of the developing nip area (which will be referred to hereinbelow as “developing nip center”). The magnetic flux density becomes lower than in the developing nip center, in both ends of the developing nip area (hereinbelow, the end on the upstream side of the developing roller's direction of rotation will be called “developing nip front end” and the end on the downstream side of the developing roller's direction of rotation will be called “developing nip rear end”).
In the first prior art, the magnet roller has a single main pole magnet having a dissimilar magnetic pole at each end on the outer and inner sides. Also, the photoreceptor drum includes a magnet therein, which is arranged so that its magnetic pole on the magnet roller side has a polarity opposite the magnetic pole on the outer side of the magnet roller. With this arrangement, it is possible to easily form a magnetic brush by a developing roller having only a single main pole magnet therein (see Japanese Patent Application Laid-open Sho 63-52167).
In the second prior art, in the developing unit, a magnetic element is arranged at a position inside the photoreceptor drum opposing the main pole magnet of the developing roller. With this arrangement, it is possible to widen the developing nip area by increasing the magnetic flux density (see Japanese Patent Application Laid-open Hei 04-287061). When the magnetic force of the main pole magnet is too strong, the rubbing force of the magnetic brush formed of the dual-component developer becomes too strong, so that the toner on the photoreceptor drum surface is disturbed by the strong rubbing force, causing image degradation of the resultant image.
Conversely, when the magnetic force of the main pole magnet is too weak, the carrier that is attracted to the photoreceptor drum by electrostatic force in the developing nip rear end the will not return to the developing roller surface but remains developed on the photoreceptor drum surface, hence transfer failure of the toner image occurs in the transfer stage, producing white voids in the resultant image.
In the recent image forming apparatus using fine particulate carrier, in order to form images of high quality, the magnetic force of the main pole magnet toward the developing nip area is weakened or the saturation magnetization of the carrier is lowered. However, when the magnetic brush is made soft so as to avoid disturbance of the toner image developed on the photoreceptor drum from the electrostatic latent image, such measures weaken the developing roller's force for collecting the carrier at the most downstream side of the developing nip with respect to the developer roller's direction of rotation, hence giving rise to the problem that the carrier is developed on the photoreceptor drum surface, causing image defects such as white voids etc. due to transfer failure.